I am a ling term owner of my HDJ100 4.2 TD and have been toying with the idea of a mild improvement for a number of years. Lockdown fever has given me some time to consider options and look at the facts. I am not an automotive engineer and would like to get input from ‘those in the know’
1) How the factory set up works
The ECU is mapped from factory but has a number of adjustments built in such as short term fuel trim and long term fuel trim. These allow based on the operating conditions (say outside temperature which influences air density) to adjust fueling to maintain the target Air Fuel Ratio (AFR).
The factory map may be common to all countries and may be set conservatively (e.g. running rich especially at low rpms) to accomdate lower quality fuels (is that correct or a myth ?)
Engines produce peak power when the stochiometric ratio is optimun which for a diesel equals an air to fuel of 14.5:1
Adding fuel without adding air makes the mixture rich but doesn’t increase efficiency. You get marginally more power but produce more smoke and mpg goes down. Rich is safe for an engine perspective bit wasteful on fuel. Reducing richness to optimal afr increases power assuming the fuel is good enough to avoid potentially damaging lean situation.
2) what to do ?
Turbocharging is interesting in the sense that adding boost allows more air into the engine, enabling extra fuel and assuming you hit optimal afr then you create more power.
Staying within the limits of the stock set-up (injectors and turbo), it seems the most effective idea is a waste gate adjustment or a boost controller. Both would allow to increase the max boost provided by the turbo which in turn will provide more air to the engine when it is fully spooled up. There are limitations to the stock turbo (see graph)
But it seems that boosting rpm by 20% sort of still is within the reasonnable efficency point. That will however only work at top end boost. To affect the low end one may want to modify the turbo (different intake turbine to spool up faster etc).
However, on the exhaust side, running higher pressure will lead to increased temperatures. The stock turbo downpipe is an interesting point here because it acts as a bottle neck and also includes the catalyst which tend to suffer from heat which exceeds design paramters.
Replacing the downpipe for a wider diameter (from stock 2.5’ to 3’) and using sport cats which offer less flow resistance seems to be the correct way to avoid excessive exhaust gas temperatures.
So increasing boost by modifying the wategate (stiffer spring or boost controller) and avoiding excessive heat buildup through new downpipe looks like a cheap and effective way to probably pick up 10-20% extra power within the limits of the stock system.
Better intercooler can be added to add cooler / denser air and obviously bigger turbo but then you need to upgrade injectors and at that point the stock ECU is also running out of adjustment headroom. At that point, you probably need a full motec set up etc... not my objective
3) Piggy back units
For years I have been fascinated by these units like unichips. There are tons of dyno graphs and you tube videos.
Fundamentally I see how such unit can alter ecu signals to adjust fueling given various circumstances. But as long as no extra air is provided, i am not sure how much real effectiveness there is. What I can see is:
- they take out excess richness at low rpms which gives more power (and better mpg)
- they add more fuel which gives slightly more power but at less optimal AFR so efficiency goes down. Black smoke isn’t my cup of tea
- they can fuel to full throttle when you are only using part throttle giving the impression of more power
- they can be used to adapt the factory ecu to run a non stock set up (eg bigger turbo, intercooler etc).
While the piggy back will modify stock ecu instructions to the injectors, it will not affect the read the stock ecu makes from the lambda sond (I think, may be I am wrong). So the ECU may want to correct what it will perceive as overfuelling. So the piggy back unit will need to be set up so it can counteract that on a dynamic basis otherwise one gradually comes back to the previous situation.
Note: that may not necessarily be true in all cases as i understand that in wide open throttle position, the ecu may stop checking paramters. But we are not flat out all the time.
Where I stand ?
For a mild improvement, I think I will go for downpipe and wastegate mod. This is a low cost solution (less then 100gbp if diy and removing cats, more expensive if sport cats are used) I can understand how it works and stock ECU should be able to manage. Essentially that means that every day will be a nice cold day where the air is denser so more is available for combustion.
Adding a unichip is definitvely a different spend. The units can be bought for diy install but retail at 1000 gbp and you rely on the default map. Given the above, I see how this could correct some excess richness but then you will simply dumping more fuel. So you get into the diminishing returns and lower efficency / mpg
You need to find a unichip tuner to adapt the mapping to your own engine and with the car in france where I can’t find unichip dealers that is not practical.
Would like to hear the view from the TD experts on the forum.
1) How the factory set up works
The ECU is mapped from factory but has a number of adjustments built in such as short term fuel trim and long term fuel trim. These allow based on the operating conditions (say outside temperature which influences air density) to adjust fueling to maintain the target Air Fuel Ratio (AFR).
The factory map may be common to all countries and may be set conservatively (e.g. running rich especially at low rpms) to accomdate lower quality fuels (is that correct or a myth ?)
Engines produce peak power when the stochiometric ratio is optimun which for a diesel equals an air to fuel of 14.5:1
Adding fuel without adding air makes the mixture rich but doesn’t increase efficiency. You get marginally more power but produce more smoke and mpg goes down. Rich is safe for an engine perspective bit wasteful on fuel. Reducing richness to optimal afr increases power assuming the fuel is good enough to avoid potentially damaging lean situation.
2) what to do ?
Turbocharging is interesting in the sense that adding boost allows more air into the engine, enabling extra fuel and assuming you hit optimal afr then you create more power.
Staying within the limits of the stock set-up (injectors and turbo), it seems the most effective idea is a waste gate adjustment or a boost controller. Both would allow to increase the max boost provided by the turbo which in turn will provide more air to the engine when it is fully spooled up. There are limitations to the stock turbo (see graph)
But it seems that boosting rpm by 20% sort of still is within the reasonnable efficency point. That will however only work at top end boost. To affect the low end one may want to modify the turbo (different intake turbine to spool up faster etc).
However, on the exhaust side, running higher pressure will lead to increased temperatures. The stock turbo downpipe is an interesting point here because it acts as a bottle neck and also includes the catalyst which tend to suffer from heat which exceeds design paramters.
Replacing the downpipe for a wider diameter (from stock 2.5’ to 3’) and using sport cats which offer less flow resistance seems to be the correct way to avoid excessive exhaust gas temperatures.
So increasing boost by modifying the wategate (stiffer spring or boost controller) and avoiding excessive heat buildup through new downpipe looks like a cheap and effective way to probably pick up 10-20% extra power within the limits of the stock system.
Better intercooler can be added to add cooler / denser air and obviously bigger turbo but then you need to upgrade injectors and at that point the stock ECU is also running out of adjustment headroom. At that point, you probably need a full motec set up etc... not my objective
3) Piggy back units
For years I have been fascinated by these units like unichips. There are tons of dyno graphs and you tube videos.
Fundamentally I see how such unit can alter ecu signals to adjust fueling given various circumstances. But as long as no extra air is provided, i am not sure how much real effectiveness there is. What I can see is:
- they take out excess richness at low rpms which gives more power (and better mpg)
- they add more fuel which gives slightly more power but at less optimal AFR so efficiency goes down. Black smoke isn’t my cup of tea
- they can fuel to full throttle when you are only using part throttle giving the impression of more power
- they can be used to adapt the factory ecu to run a non stock set up (eg bigger turbo, intercooler etc).
While the piggy back will modify stock ecu instructions to the injectors, it will not affect the read the stock ecu makes from the lambda sond (I think, may be I am wrong). So the ECU may want to correct what it will perceive as overfuelling. So the piggy back unit will need to be set up so it can counteract that on a dynamic basis otherwise one gradually comes back to the previous situation.
Note: that may not necessarily be true in all cases as i understand that in wide open throttle position, the ecu may stop checking paramters. But we are not flat out all the time.
Where I stand ?
For a mild improvement, I think I will go for downpipe and wastegate mod. This is a low cost solution (less then 100gbp if diy and removing cats, more expensive if sport cats are used) I can understand how it works and stock ECU should be able to manage. Essentially that means that every day will be a nice cold day where the air is denser so more is available for combustion.
Adding a unichip is definitvely a different spend. The units can be bought for diy install but retail at 1000 gbp and you rely on the default map. Given the above, I see how this could correct some excess richness but then you will simply dumping more fuel. So you get into the diminishing returns and lower efficency / mpg
You need to find a unichip tuner to adapt the mapping to your own engine and with the car in france where I can’t find unichip dealers that is not practical.
Would like to hear the view from the TD experts on the forum.